1. Field of the Invention
The present invention relates to audio signal transmission systems in general and in particular to the use of an RC network across the output of an audio signal source or across the input of an audio signal transmission cable coupled to such a source for reducing audible noise in such systems.
2. Description of the Prior Art
An audio signal transmission system of the type to which the present invention relates comprises an audio signal source, an audio signal transmission cable and a load. For example, the audio signal source may comprise an audio signal pre-amplifier, a power amplifier, a transducer or the like for providing signals in the audio frequency spectrum; the transmission cable may comprise a pair of parallel electrically insulated wires or a coaxial cable; and the load may comprise a pre-amplifier, a power amplifier, a speaker or other load depending on the source.
The audio frequency spectrum is generally considered to encompass a frequency range of from 20 Hz to 20,000 Hz.
Various means have been proposed for suppressing noise in audio signal transmission systems. For example, in U.S. Pat. No. 4,177,431 an interface circuit is provided for suppressing noise in a circuit comprising a power amplifier and a speaker coupled thereto by a "superspeaker" cable. The interface circuit, comprising a series coupled capacitor and resistor, is coupled between the superspeaker cable, which has low characteristic impedance, e.g. 9 ohms, and the speaker, which also has a low characteristic impedance of 4 to 16 ohms over the audio frequency range.
The patented interface network is provided for terminating the superspeaker cable at high frequencies, or in other words, to prevent spurious amplifier oscillations caused by reflections of high frequency signals due to the effect of the inductive portion of the loudspeaker impedance at high frequencies. The frequency of the spurious oscillations was found to be on the order of 1 megaHertz (1 mHz). To properly terminate the cable at that frequency, typical values chosen for the series resistor and capacitor were 6.2 ohms and 0.05 .mu.F, respectively.
While possibly effective for suppressing 1 mHz noise caused by standing waves due to reflections on a transmission cable between a power amplifier and a speaker, the patented interface circuit does not address the problem of audio frequency noise.
Another disadvantage of the patented interface circuit concerns the fact that the suggested capacitance in the patented circuit is large and the resistance value is so small that instability and spurious oscillation of the power amplifier may occur.
It is well understood that under certain circumstances filter circuits coupled to the output of an amplifier having a resonant frequency outside the bandwidth of the amplifier to which they are coupled can cause the amplifier to oscillate. The resonant frequency associated with the RC time constant of the patented interface circuit is clearly outside the bandwidth of most audio frequency power amplifiers, and at the very limits of the more expensive commercial models which typically have a bandwidth of 20 Hz to 1 MHz.